Aromatic nitro compounds are broadly useful intermediates. They can be synthesized in a number of ways, but the most common method employs mixtures of nitric and sulfuric acids. In operating this method however, a spent sulfuric acid containing water is formed from the nitration process and it must be reclaimed if large scale nitration is to be at all economical. Since recovery of spent sulfuric acid is expensive, primarily due to energy costs associated with the removal of water, other methods of nitration are of continued interest.
One alternate method involves using a trifluoroacetic acid medium in place of sulfuric acid. Trifluoroacetic acid has a boiling point of 72.degree. C. Because of this low boiling point, the acidic solvent for the nitration reaction can be recovered by distillation using exhaust steam as a source of heat.
An important consideration in choosing a particular medium for nitrating aromatics is the ability to carry out the nitration reaction using lower valent; N(III) and N(IV); nitrogen compounds. By using a nitrogen oxide, less H.sub.2 O is produced during the nitration process allowing for easier separation of the reaction products. N(III) and N(IV) oxides are more readily available and less expensive than N(V) oxide.
U. A. Spitzer and R. Stewart, J. Org. Chem., 1974, 39, 3936 disclose nitrating aromatics in the presence of a trifluoroacetic acid medium. In one example benzene and sodium nitrate were reacted together in a 1:1 molar ratio at room temperature to form nitrobenzene in a 99.9% yield. In another example sodium nitrite was reacted with benzene in the same 1:1 molar concentration and at the same temperature as the nitrate example, however, nitrobenzene yield was only about 3%.
Richard O. C. Norman, et al. in J. C. S. Perkin I, 1974, 369 disclose nitrating aromatics with nitrogen dioxide in the presence of trifluoroacetic acid. In one series of runs nitrogen dioxide was bubbled through a solution of benzene in trifluoroacetic acid at temperatures from 0.degree.-100.degree. C. and with reaction times from 1-5 hours. Nitrobenzene yields ranged from about 2% to 54%. It was reported however, that adding urea to the system increased the nitrobenzene yield up to as much as 99%.
Sake Uemura, et al. in J. C. S. Perkin I, 1978, 1077 disclose nitrating aromatics using stoichiometric amounts of arenes and nitrating agents in the presence of trifluoroacetic acid at 25.degree. C. When sodium nitrate was added to benzene, or a derivative thereof, the nitroarene yields were reported to be as high as 100%. Similarly, when sodium nitrite was used in the 3:1 stoichiometric ratio, instead of sodium nitrate, under the same reaction conditions, nitroarene yields were again reported to be as high as 100%. When the reaction was run under the conditions employed by Spitzer and Stewart; 1:1 molar ratio of arene to nitrating agent in 340 mmol of trifluoroacetic acid at 25.degree. C.; nitroarene yield was reported at only 20%.